The present invention relates to an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus and a developing agent used in the apparatus and, more particularly, to an image forming apparatus for performing development by the magnet rotating development method and a two-component developing agent containing magnetic toner and carrier for use in the apparatus.
An image forming apparatus using the magnet rotating development method is disclosed in, e.g., Jpn. Pat. Appln. KOKOKU Publication No. 7-40156. In this apparatus, a developing roller facing an image carrier for carrying an electrostatic latent image is composed of a hollow cylindrical rotary sleeve made of a nonmagnetic material and a magnetic roll formed inside this sleeve, said magnetic roll having a plurality of magnetic poles and being able to rotate independently of the sleeve.
In the magnet rotating development method, a two-component developing agent containing magnetic carrier and magnetic toner formed by adding a magnetic material to the surfaces of toner particles is applied to the developing roll. The magnetic roll and the sleeve are rotated in the same direction or in opposite directions to convey the developing agent to a development region while the developing agent itself is rotated. In this magnet rotating development method, control of the specific toner density is easier than in the common magnet fixed development method, so the specific toner density can be increased. This increases the toner conveyance amount and raises the development efficiency. When the magnetic roll and the sleeve are rotated in the same direction, the rotating direction of the developing agent is opposite to its conveyance direction. When the magnetic roll and the sleeve are rotated in opposite directions, the rotating direction of the developing sleeve is the same as its conveyance direction. This is suitable for high-speed development because the conveyance amount can be further increased.
In two-component development by the magnet fixed development method, the specific toner density on a developing roll is commonly about 6 wt %. However, in two-component development by the magnet rotating development method, the specific toner density stays around about 50 wt %. The result is the advantage that so-called beads carry over which is a phenomenon in which the carrier on a developing roll adheres to an image carrier hardly occurs. In the magnetic fixed development method, to prevent this carrier transfer and control the charge amount, the surface of a magnetic carrier is coated with a silicon-based coating material.
Unfortunately, in magnetic toner used in the magnet rotating development method, a magnetic powder is mixed in and adhered to the toner in an amount of 4.5 wt % with respect to the toner weight. Although high-quality images can be provided initially, the externally added magnetic powder causes some inconveniences.
Representative inconveniences are the following three phenomena.
The first is an increase in wear of the surface of a photoreceptor due to the externally added magnetic powder. When the surface of a photoreceptor wears, its sensitivity lowers, so no stable electrostatic charge can be provided any longer. This reduces the life of the photoreceptor.
The second is an increase in wear of a cleaning blade due to the externally added magnetic powder. When a cleaning blade wears, inferior cleaning takes place, leading to deterioration of images.
The third is an increase in wear of the surface film of a heat roller due to the externally added magnetic powder. When the surface film of a heat roller wears, high-temperature offset occurs. This reduces the life of the heat roller.
These inconveniences significantly reduce the prescribed life of an apparatus when the externally added magnetic powder is used.
In contrast, toner containing no magnetic powder effectively eliminates the above inconveniences. However, at high temperatures and high humidities conventional magnetic carrier such as a Cu--Zn carrier causes inferior electrostatic charge. Accordingly, image density lowers when a developing agent containing such carrier is used. Also, when highly chargeable magnetic carrier is used, the chargeability of the toner becomes too high. Consequently, image density lowers when sheets are continuously fed. Furthermore, highly chargeable magnetic carrier has small electrical resistance and magnetic force margins and hence is difficult to rationalize.